1. Field of the Invention
The present invention relates to a field sequential driving type liquid crystal display (FS-LCD) and, more particularly, to a method of driving an analog driving type LCD capable of improving a response speed.
2. Description of Related Art
A color LCD generally includes a liquid crystal panel including an upper substrate, a lower substrate, and a liquid crystal injected between the upper and lower substrates, a driving circuit for driving the liquid crystal panel, and a backlight for providing white light or other color lights to the liquid crystal. Such a color LCD may be mainly classified into a color filter type or a color field sequential driving type based on the manner in which it is driven.
The FS-LCD allows red (R), green (G), and blue (B) backlights to be arranged in one pixel that is not divided into R, G, and B subpixels, wherein light of the three primary colors is provided from the R, G, and B backlights to one pixel through the liquid crystal so that they are sequentially displayed in a time division manner, thereby displaying a color image using a residual effect.
The color FS-LCD sets a plurality of reference voltages corresponding to the number of gray scales to be displayed, and one reference voltage corresponding to the gray scale data among the plurality of reference voltages is selected using an analog switch, and the selected reference voltage drives the liquid crystal panel, and the gray scale is displayed by the amount of transmitted light corresponding to the applied voltage.
FIG. 1 is a diagram for explaining a conventional analog method of driving an LCD, which shows waveforms for explaining a method of driving the LCD to display the gray scales by varying the driving voltage of the liquid crystal. FIG. 1 shows the driving voltages applied to the liquid crystal and the corresponding waveforms with respect to the amount of light transmitted through the liquid crystal.
Referring to FIG. 1, a driving voltage of V11 level is applied to the liquid crystal during a period (T1) from t1 to t3 in time, and light corresponding to the driving voltage of V11 level is transmitted through the liquid crystal. A driving voltage of V12 level, which is higher than V11 level, is applied during a period (T2) from t4 to t6, and the amount of transmitted light corresponding to the driving voltage of V12 level is obtained. A driving voltage of V13 level which is higher than V11 and V12 levels is applied during a period (T3) from t7 to t9, and the amount of transmitted light corresponding to the driving voltage of V13 level is obtained.
In effect, R color is displayed during a period Tr from t2 to t3 in which an R light emitting diode of the R backlight emits light, G color is displayed during a period Tg from t5 to t6 in which a G light emitting diode of the G backlight emits light, and B color is displayed during a period Tb from t8 to t9 in which a B light emitting diode of the B backlight emits light.
Such an analog driving method of varying the driving voltage has a problem in that the response speed of the liquid crystal is slow due to a delayed falling time of the liquid crystal. In addition, it is difficult to implement time-varying images due to the decreased response speed of the liquid crystal.
Methods for coping with the above-mentioned problem by displaying the gray scale by means of digital control are disclosed in JP Patent Publication Nos. 2003-98505, 2003-099015, and 2003-107425.
One of the digital gray scale display methods includes storing voltage-applied times corresponding to the gray scales in a look-up table, reading out the voltage-applied time corresponding to the gray scale data from the look-up table, and applying a constant voltage to the liquid crystal during the voltage-applied time read from the look-up table to display the gray scale. The method includes making constant the driving voltage applied to the liquid crystal and controlling the voltage-applied time to display the gray scale. Accordingly, the response speed of the liquid crystal in response to the gray scale level may be improved by making the driving voltage constant and controlling voltage applied state and non-voltage applied state in a timing manner.
Another method for displaying the gray scale by means of digital control includes storing applied patterns corresponding to the gray scales in a look-up table, reading out the applied patterns corresponding to the gray scale data from the look-up table, and applying driving voltages of constant levels to the liquid crystal in response to the applied patterns read out from the look-up table during a unit period of emitting light of the light emitting diode to display the gray scale. This method includes varying the applied patterns during the unit period of emitting light of the light emitting diode to control voltage applied state and non-voltage applied state in a timing manner. Accordingly, the response speed of the liquid crystal may be improved by displaying the gray scale in response to the voltage-applied time.
Yet another method for displaying the gray scale by means of digital control includes corresponding each area which has integrated waveforms of light transmitting the liquid crystal with light emitting periods of the light emitting diode (LED) to each gray scale when the driving voltage is applied to the liquid crystal, and varying the areas to display the gray scales.
According to the above-mentioned method of integrating the transmitted light, the voltage-applied time is set in consideration of areas which have integrated waveforms of light transmitted through the liquid crystal with light emitting periods of the LED, so that a fine gray scale suitable for displaying the gray scale may be implemented, and the waveforms of transmitted light are rapidly falling and rising, thereby improving the response speed of the liquid crystal.
FIG. 2 shows waveforms for explaining a method of driving the conventional digital driving type LCD, which shows waveforms in response to driving data of predetermined bits and resultant waveforms of the amount of light transmitted through the liquid crystal.
Referring to FIG. 2, driving data corresponding to each gray scale are provided as digital signals of predetermined bits, for example, 7 bits, and the driving voltage corresponding to the driving data of 7 bits is applied to the liquid crystal. The applied driving voltage determines the amount of light transmitted through the liquid crystal to display the gray scale.
However, the number of bits of the driving data in the above-mentioned conventional digital driving type should increase in order to display the full color gray scale at a fast response speed. In the meantime, the FS-LCD sequentially drives R, G, and B LEDs in a time-sharing manner as compared to a general LCD, so that it has a driving frequency higher than that of the general LCD. Accordingly, when the number of bits of the driving data increases in order to display the full color gray scale at a fast response speed, the driving frequency should also be increased.
As this driving frequency increases, a problem arises that image quality is deteriorated due to distortion resulting from a gate driving voltage and a common power source Vcom. In addition, the liquid crystal is fast driven by a high driving frequency, which causes the power consumption to be increased. Further, in accordance with the conventional digital driving type, the gray scale to be currently displayed has an effective value response different from that of the gray scale that has been displayed just before, which causes the gray scale not to be exactly displayed. In particular, when an intermediate gray scale is required to be displayed, the influence of the gray scale that has been displayed just before on the gray scale to be currently displayed is further increased.
As such, a method for displaying the gray scale using a reset pulse for coping with the problem of the conventional driving type that the effective value response is changed due to the gray scale that has been displayed just before, is disclosed in U.S. Pat. No. 6,567,063.
FIG. 3 shows waveforms for explaining a conventional method for displaying digital gray scales using reset pulses. Referring to FIG. 3, a plurality of periods T31-T36 are periods in which R, G, and B LEDs for R, G, B backlights are driven to display the gray scales with respect to R, G, and B colors per each of the periods.
A predetermined voltage VLC corresponding to R gray scale data is applied to the liquid crystal in the period T31, and light is transmitted through the liquid crystal in response to the applied voltage, so that R light is displayed in a period where the R LED (RLED) emits light. A predetermined voltage VLC corresponding to G gray scale data is applied to the liquid crystal in the period T32, and light is transmitted through the liquid crystal in response to the applied voltage, so that G light is displayed in a period where the G LED (GLED) emits light. In the meantime, a predetermined voltage VLC corresponding to B gray scale data is applied to the liquid crystal in the period T33, and light is transmitted through the liquid crystal in response to the applied voltage, so that B light is displayed in a period where the B LED (BLED) emits light. Accordingly, a color having desired gray scales is displayed.
In accordance with the above-mentioned digital driving type, a predetermined voltage is applied, which has a different absolute value from that of the gray scale data and is irrelevant to the gray scale data during each predetermined time t31-t36 at each ending point of the periods T31-T36. Accordingly, R, G, and B colors having predetermined gray scales are displayed during each of the periods T31-T36 and the voltage that is irrelevant to the gray scale data is provided at each ending point of the periods so that no light may be transmitted. Accordingly, when the liquid crystal is driven by the applied voltage corresponding to the gray scale data during each of the periods T31-T36, liquid crystal state as well as transmissivity in the previous period does not affect the current period, which leads to an improvement in the response speed of the liquid crystal. In this case, the applied signal at an ending point of each period T31-T36 is referred to as a reset pulse, which improves the response speed of the liquid crystal.
Accordingly, the above-mentioned digital gray scale display method advantageously improves the response speed of the liquid crystal to implement dynamic images. However, in the digital gray scale display method, predetermined bits of driving data should be allocated to the number of reset pulses, so that the number of driving data bits is further increased as compared to the typical digital driving type. As the number of driving data bits increases, the driving frequency increases, which leads to an increase in power consumption as mentioned above, so that the problem of deteriorating the image quality due to distortion of the gate voltage and common voltage is still present.
As a result, when the LCD is driven in the above-mentioned digital manner, the gate pulse width having a threshold value or more should be maintained, which causes the driving speed to be limited, and also limits an increase of a frame frequency for preventing flicker from occurring. Accordingly, an inversion-driving type for improving the image quality cannot be applied, which results in problems such as crosstalk, flicker, and so forth.